Single crystal casting and solidification of molten metal using a ceramic investment shell mold positioned on a chill plate is well known in the art of investment casting to produce such components as single crystal gas turbine engine blades and vanes. In one mode of single crystal casting, the ceramic shell mold includes a lower grain starter cavity having a single crystal seed positioned therein. The seed is communicated to an upper mold cavity. The seed is placed in or otherwise communicated to the lower starter cavity of a ceramic shell mold that is preformed using the well known lost wax process. When molten metal is introduced into the shell mold, a single crystal is expected to grow epitaxially upwardly from the seed with a preselected crystallographic orientation determined by the seed for propagation through the molten metal in the mold cavity. During single crystal solidification, the melt-filled shell mold is withdrawn from a casting furnace to establish unidirectional heat removal from the molten metal in the mold to produce a solidification front of the single crystal through the molten metal in the mold cavity to form the single crystal casting.
Single crystal seeds used heretofore in casting nickel base superalloys have employed seed alloy compositions similar to that of the nickel base superalloy to be cast. Such seeds can experience oxide growth on the seed surfaces at the elevated mold preheat temperatures employed in single crystal casting. This oxide growth occurs while preheating the mold in a vacuum casting furnace and can result in nucleation of spurious grains that grow in the starter cavity instead of the expected single crystal. Such spurious grain nucleation and growth in the starter cavity can result in an increase in scrap castings by virtue of their having a grain orientation outside a preselected specification (e.g. where the [001] crystal axis must be within a certain range of degrees relative to the z axis of the turbine airfoil).
An object of the present invention is to provide a single crystal seed as well as casting mold and method for making a single crystal casting that overcome the above problem of oxide growth and spurious grain nucleation and the need for special gating schemes.